Alzheimer's Disease is a devastating neurodegenerative disorder. Its sporadic forms affect an elderly population (sharp increase in incidence at >75 years of age), in addition, there are various familial forms with an onset of the disease in the fourth or fifth decade of life. Pathologically, it is characterized by the presence of extracellular senile plaques, and intracellular neurofibrillar tangles in patient's brains. The core constituent of the senile plaques are small, 4 kDa amyloid peptides. They are generated by the proteolytic processing of a large transmembrane protein, amyloid precursor protein (APP). Cleavage of APP by beta-secretase (BACE-1) releases the soluble APP-beta fragment, while the 99-amino acid long C-terminus remains tethered to the membrane. This C-terminal fragment is subsequently proteolytically processed by gamma-secretase (an membrane multi-enzyme complex) to generate amyloid peptides of various length, predominantly 40 and 42 amino acids long (Hardy J, Selkoe D J (2002) Science; 297 (5580):353-356).
If, under pathologic conditions, the generation of these peptides occurs at an increased rate, or if their removal from the brain is disturbed, increased brain amyloid peptide concentrations leads to the formation of oligomers, fibrils and eventually plaques (Farris W, et al (2007) Am. J. Pathol.; 171 (1):241-251). It has been shown, that deposition of amyloid peptides and plaques in the brain is the first measurable event in the pathogenesis of Alzheimers Disease, and that it is the trigger for loss of synapses, synaptic contacts, and neurons (Grimmer T, et al (2009) Neurobiology of Aging; 30 (12):1902-1909). Brain atrophy caused by massive neuron loss is followed by impairments in cognition, memory, orientation and the ability to perform the tasks of daily living, i.e. clinically manifest dementia (Okello A, et al (2009) Neurology; 73 (10):754-760).
BACE-1, also known as Asp2 or Memapsin 2, is a transmembrane aspartic protease highly expressed in neurons. It co-localizes with its substrate APP in Golgi and endocytic compartments (Willem M, Lammich S, Haass C (2009) Semin. Cell Dev. Biol; 20 (2):175-182). Knock-out studies in mice have demonstrated the absence of amyloid peptide formation, while the animals are healthy and fertile (Ohno M, et al (2007) Neurobiol. Dis.; 26 (1):134-145). Genetic ablation of BACE-1 in APP-overexpressing mice has demonstrated absence of plaque formation, and the reverse of cognitive deficits (Ohno M, et al (2004) Neuron; 41 (1):27-33). BACE-1 levels are elevated in the brains of sporadic Alzheimer's Disease patients (Hampel H, Shen Y (2009) Scand. J. Clin. Lab. Invest.; 69 (1):8-12).
Taken together, these findings suggest that the inhibition of BACE-1 may be a favourable therapeutic strategy for the treatment of Alzheimer's Disease.
Beta-site amyloid precursor protein cleaving enzyme 2 (BACE-2) is a transmembrane aspartic protease that is highly expressed in pancreatic β cells and other peripheral tissues (Bennett B, et al (2000) JJ. Biol. Chem. 275(27) 20647-20651). BACE-2 is closely related to BACE-1 or beta secretase. However, despite structural and sequence similarities the substrate specificity of BACE-1 and BACE-2 appear to be different. While Aβ or β-amyloid peptide is the main substrate of BACE-1, BACE-2 does not generate either form of Aβ (Vassar R, et al (1999) Science 286, 735-741).
Transmembrane protein 27 (TMEM27 or collectrin) plays an important role in β-cell proliferation and insulin secretion (Akpinar P, et al (2005) Tmem27: Cell Metabolism. 2(6) 385-397) and has been identified as a substrate for BACE-2 (WO 2010/063718). Tmem27 exists as a dimer and the extracellular domain is cleaved and shed from the plasma in a β cell-specific manner. Overexpression of full-length Tmem27, but not the truncated or soluble protein, increases β cell proliferation, suggesting that the full length protein is required for this biological function. Tcf1 (hepatocyte nuclear factor-1α, HNF-1α) controls the transcription of TMEM27. Mice with targeted deletion of Tcf1 exhibit decreased β cell mass, and knockdown of Tmem27 using RNAi results in a reduction of cell proliferation. Transgenic mice with increased expression of Tmem27 in pancreatic β cells exhibit increased β cell mass compared to their wild-type littermates. This data indicates that TMEM27 plays a role in control of β cell mass and that inhibition of BACE-2 which cleaves TMEM27 could be useful for treating loss of β cell mass and function, the underlying cause of diabetes.
Taken together, these findings suggest that the inhibition of BACE-2 may be a favourable therapeutic strategy for the treatment and prevention of metabolic disorders related to decreased β cell mass and/or function, such as type 2 diabetes.